Thermal ink jet printing is generally a drop-on-demand type of ink printing system which uses thermal energy to produce a vapor bubble in an ink filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electric pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink. As the bubble begins to collapse, the ink in the channel between the nozzle and the bubble starts to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum of the droplet which proceeds in a substantially straight line towards a recording medium, such as paper or transparencies, etc. (hereinafter, referred to as a "sheet"). Sheets are moved past the printhead for creation of the image.
In one type of ink jet device, a printhead including an array of several nozzles oriented parallel to the direction of sheet travel is moved across the recording medium in a direction perpendicular to the direction of sheet travel. Each transit prints a swath of spots or pixels across the page corresponding to an image. Most thermal ink jet printers print some number of scanlines in a swath, and the swath boundaries are visible in prints made thereby. The boundaries are visible because of ink flowing beyond the boundary of the array end (in the case of paper) or pulling away from the boundary of the array end due to surface tension (in the case of transparencies). In the former case a thin line along the boundary is more heavily inked; in the latter it is not inked at all.
It will be clear from the above description that printing can be accomplished in each transit of the head across the page, and for black (or monocolor) only printing, bidirectional printing is common. Speed of printing a page is at least partly a function of the number of scan lines that can be printed in a single transit across the page. In operation, an ink jet printer prints a number of scan lines in a first transit, relative motion between the printhead and sheet is provided to advance the printhead with respect to sheet, and then the ink jet head prints a second number of scan lines in a second transit. Bidirectional printing can greatly increase the speed of printing. Less common, is bidirectional printing for color ink jet system. Color ink jet printing is normally performed unidirectionally (left to right or right to left, not both). Multiple nozzle arrays arranged in parallel may be provided to print with multiple inks printing plural separations. One particular type of color ink jet printer has a single head (corresponding to an array of nozzles) per color, with four heads mounted one behind the other on the carriage, so that all four colors cyan, magenta, yellow and black of all scanlines in a swath are printed in a single pass, with a fixed order for the separations within a pixel. U.S. patent application Ser. No. 08/208,556, entitled "Bidirectional Color Ink Jet Printing with Head Signature Reduction", by R. V. Klassen, details such a device with bidirectional operation.
Due to the large number of variables in carriage movement and ink deposit, a forward transit cannot match a return transit precisely. Slight discontinuities occur between bidirectional swaths. If, for example, a line is printed on a page arranged parallel to the ink jet array, with a length bridging one or more swaths, and bidirectional printing is used, the line will be slightly offset from swath to swath. As detailed in U.S. Pat. No. 5,044,796 to Lund, image quality concerns occasionally suggest that certain swaths should be printed unidirectionally. Data forming a swath is checked for breaks, i.e., white areas, or no structures bridging the line. If such an area is found, and it is more than about 3/4 of the printhead height, then only the data down to the break is imaged, and the remaining data is printed in the next swath. The reverse transit of the head always occurs at a break.
As a printhead becomes larger however, which is desirable from a speed point of view, it becomes more difficult to find breaks in the document. The result is that more and more swaths are printed unidirectionally, resulting in failing to take advantage of the potential speed advantage offered by the larger printhead.
References disclosed herein are incorporated by reference for their teachings.